Existing deformable mirror technologies make use of a monolithic substrate to support the mirror face sheet, which is either bonded to the substrate or is an integral part of the substrate itself.
FIG. 1A shows a prior-art surface normal actuator (SNA) design, and FIG. 1B shows a prior-art surface parallel actuator (SPA) design. In FIG. 1A, the deformation of the mirror face sheet 102 is achieved using actuators 104 which react against a rigid back plane 106. In FIG. 1B, the actuators 114 are parallel to the mirror face sheet 112. The actuators 114 deform the substrate structure itself, but do not require a rigid back plane.
The SNA design requires a stiff monolithic back plane, which limits the extent to which the substrate can be light weighted, especially for large aperture mirrors. The SPA design, on the other hand, requires the deformation of the monolithic substrate itself, resulting in high stress levels and stress concentrations in the substrate, which limit the extent to which the substrate can made lightweight. Typically, the SNA design is heavier than the SPA design, but it can provide optical correctability to higher spatial frequencies; as such, more actuators can be used per unit surface area of the mirror.
The stresses induced in the mirror face sheet itself, due to bending and hoop stresses, also influence the design and the achievable weight. Past and current deformable mirror technologies have typically used glass face sheets, which cannot be thinned below about 1 mm. New mirror face sheet technologies such as the nano-laminate and membrane approaches are producing extremely thin face sheets, typically less than about 0.05 mm, which are extremely light (small fractions of kg per square meter), and which also have extremely low bending stiffness.
There is also a hybrid deformable mirror technology under development that combines the nano-laminate technology with the monolithic substrate SPA technology. This combination is expected to push mirror weight down to values smaller than the current glass technologies, but much heavier than the nano-laminate itself. If it becomes necessary to push the light weighting down further, a further evolution of substrate and actuation technologies would be required to enable extremely large aperture designs, for a given mirror weight.
There are research efforts aimed at eliminating the distributed support (substrate) structure altogether and deform the mirror face sheet with some distributed actuation scheme such as distributed “patch” actuators or electrostatic actuation using some charge deposition scheme using electron gun. However, all such schemes suffer from a fundamental difficulty due to the lack of rigidity of the mirror. In addition to being able to deform the mirror, it is necessary to provide some rigidity to the mirror, not only to withstand disturbances but also to ensure that the response to a given set of actuator commands is unique.
A major design constraint in the prior art mirror designs is the effect of the Coefficient of Thermal Expansion (CTE) mismatch between the face sheet and its supporting substrate, which drives the design to (a) minimize CTE mismatch, or reduce CTE to almost zero, and (b) impose temperature control with tight tolerances.